In response to the ever increasing demand in transmission capacity, telecommunication systems for operation at 100 Gb/s are already under development. An important challenge at such high speed transmission is the spectral efficiency required to increase the transmission capacity over existing transmission is links. New approaches based on coherent detection appear as the most promising. They enable polarization multiplexing and the mitigation of transmission impairments through digital signal processing in the electrical domain.
In coherent detection, the optical signal is demodulated by mixing with a reference, the ensuing beats being detected by photodiodes [K. Kikuchi, “Coherent optical communication systems,” Chapter 3 of Optical Fiber Telecommunications V, Volume B, by I. P. Kaminow, T. Li and A. E. Willner, Elsevier (2008)]. The resulting electrical signals are further digitized and processed in the electrical domain. The mixing and detection are achieved using an assembly of optical and optoelectronics components such as shown in FIG. 1. This assembly is referred to as the optical front-end (OFE) of the coherent receiver.
The purpose of the optical front end illustrated in FIG. 1 is to provide four electrical signals allowing the determination of the amplitude, phase and polarization of the optical signal Es. It separates the incoming signal Es and a reference field produced by a local oscillator ELO into x and y polarization components that are properly aligned for maximum interference and fed into two 90° optical hybrid mixers. These mixers provide in-phase and quadrature signals allowing unambiguous determination of the amplitude and phase of each polarization component Esx and Esy. The beats between the signal polarization components and the reference field are detected by photodiodes. Resulting photocurrents are amplified and converted to output voltages (Ix, Qx, Iy, Qy) using linear trans-impedance amplifiers (TIA). These voltages can then be digitized and processed to mitigate transmission impairments and decode the incoming signal.
The polarization management function is illustrated schematically by two polarization beam splitters in the drawing of FIG. 1. Actual implementation may differ. For example, the signal LO from the local oscillator is in general linearly polarized and can be separated into linearly polarized components of equal amplitude by a 3 dB splitter. Further components may be required to ensure proper alignment of the signal and reference fields. Moreover, the polarization management and mixing functions can be intertwined to some extent. For example, the reference field can be transformed into a circularly polarized field in order to ensure the quadrature condition.
In principle, two beat signals in quadrature are sufficient to unambiguously determine the amplitude and phase of an optical field. Two optical outputs from each hybrid with beats in quadrature (e.g Esx+ELO and Esx+jELO) could each be detected with a single photodiode to determine the amplitude and phase of the signal. However, important noise terms are not eliminated through this process, and careful adjustment of the signal and local oscillator powers is necessary to avoid severe system impairment [see U.S. Pat. No. 6,859,586 (EPWORTH et al) and Carena, V. Curri, P. Poggiolini and F. Forghieri, “Dynamic range of single-ended detection receivers for 100GE Coherent PM-QPSK,” IEEE Photon. Technol. Lett., 20, 1281-1283 (2008)]. The intensity resulting from the mixing of two optical fields is given by the sum of the individual field intensities and a beat signal carrying the useful phase information. Preferably, the detection process should reject the individual intensity contributions and retain only the useful beat intensities. This is realized with balanced detection as illustrated in FIG. 1. Mixed optical intensities carrying the same individual intensities but beats that are out of phase by π are detected differentially by balanced photo-detectors. Individual intensities are thus subtracted, whereas the beat intensities are added, doubling the amplitude of the meaningful photocurrent. Balanced detection thus allows using all of the received signal power for detection, while rejecting common-mode signals. Compared to single-ended detection, the use of balanced detection provides higher optical power dynamic range and longer reaches.
In balanced detection, two optical signals are detected using similar photodiodes. The resulting photocurrents are amplified differentially in order to produce an electrical signal proportional to their difference. The aim of this differential detection is to highlight the difference between similar optical signals by rejecting their common part. It is know in the art to quantify the ability of a pair of balanced photo-detectors to perform this rejection by a factor called the common mode rejection ratio (CMRR), as for example explained in G. Bach, “Ultra-broadband photodiodes and balanced detectors towards 100 Gbit/s and beyond,” Proc. of SPIE v.6014, 60140B (2005). It corresponds to the ratio of the weak signal measured under equal illumination of both detectors and the strong signal measured when a single detector is illuminated. FIGS. 2A, 2B and 2C (PRIOR ART) depicts the three illumination conditions required to measure the CMRR. Under dual-photodiode illumination with the same optical power (FIG. 2A), a weak photocurrent ΔI is measured while strong photocurrents I1 and −I2 are detected under single-photodiode illumination (FIGS. 2B and 2C). The CMRR is defined here as the ratio of these values:
                    CMRR        =                                                        Δ              ⁢                                                          ⁢              I                                                                                                    I                1                                                    +                                                        I                2                                                                                      (        1        )            
It qualifies the similarity of the photodiodes (responsivity, polarization dependence, frequency response) by quantifying the relative weakness of the output electric signal under equal illumination. The CMRR definition is simple and its measurement appears straightforward but does require some care. Nonlinearity can render the CMRR power dependent. Measurements should thus be carried out with the same power incident on each photodiode surface as illustrated in FIG. 2. Moreover, the frequency response of the photodiodes may differ, rendering the CMRR dependent on the modulation frequency of the incident power. Typically, the CMRR of balanced photodetectors is specified as a function of frequency.
One practical issue with the use of the CMRR is that photocurrents I1 and I2 of individual photodiodes cannot be measured without physically blocking the light otherwise reaching a photodiode. This is not possible, in general, when characterizing a coherent receiver OFE in which the photodiodes and the optical mixer are integrated and connected, for example using optical fibers. There is therefore a need for an improved method for characterizing the performance of a balanced detection system and an apparatus implementing such method.